The present invention relates to a method for forming a compound semiconductor layer of relatively high purity on a semi-insulating substrate.
Compound semiconductor materials, such as III-V, II-VI, and ternary and quaternary compounds, are frequently used in electronic devices which are to be operated at high frequencies. In part, this is because such materials intrinsically provide greater electron mobility than silicon or germanium. The devices are typically structured with an active monocrystalline compound semiconductor layer disposed on a substrate of semi-insulating material. A highly resistive substrate material is chosen because such materials provide electrical isolation for the device or multiplicity of devices generated thereon and because they provide good dielectric mediums for devices which are to be operated at radio or microwave frequencies.
Gallium arsenide is a frequently used substrate material because, among other things, a high quality epitaxial active layer can readily be nucleated thereon. So as to insure their semi-insulating nature, GaAs substrates are typically doped with chromium and/or oxygen. Additionally, semi-insulating GaAs substrates often contain other impurity atoms which, for example, might have been introduced during substrate fabrication.
The active semiconductor layer is typically formed by epitaxial growth on a planar surface of the substrate and it is well-known that the physical condition of the substrate surface greatly affects the quality of the epitaxial layer. In order to achieve a clean, smooth and relatively strain free surface, a variety of mechanical polishing techniques as well as liquid and gaseous etching techniques are commonly employed. For example, alternative conventional polishing techniques are described in U.S. Pat. No. 3,855,024, METHOD OF VAPOR PHASE POLISHING A SURFACE OF A SEMICONDUCTOR, M. Lim, Dec. 17, 1974, and in U.S. Pat. No. 3,808,072, IN SITU ETCHING OF GALLIUM ARSENIDE DURING VAPOR PHASE GROWTH OF EPITAXIAL GALLIUM ARSENIDE, J. V. DiLorenzo, Apr. 30, 1974.
However, I have discovered that even when a high quality semi-insulating substrate surface is provided, a subsequently grown epitaxial film is invariably contaminated. This contamination has been observed despite close processing controls during the epitaxial growth. In an effort to reduce the level of this contamination, the present invention was discovered.